Is This a Good $600 Build for Modded Kerbal Space Program?

I'm planning a roughly $600 PC mainly for general everyday tasks and heavily modded Kerbal Space Program. Since the game can be very CPU-limited and may rely heavily on a single thread, I'm prioritizing a strong processor and at least 16 GB of RAM. The graphics card is less important, so I'm considering buying a used GTX 1080 for around $80. I have a Micro Center nearby and would like to use one of its CPU, motherboard, and RAM bundles if that lowers the total cost. The listed build comes to about $608 and includes a Ryzen 5 5500X3D, 16 GB of DDR4-3200 memory, a B550 motherboard, a 512 GB NVMe SSD, a 750 W power supply, and the used GTX 1080. I also found a used complete PC for $525 with the same CPU and memory but an RTX 3060 12 GB. It might save me around $100, and I could test it before buying. Would that used system be a better deal, and should I upgrade it to 32 GB of RAM?